Directional aerial systems



Ap 1,1959 I R..F.Pwn-" 2,881,4 4

DIRECTIONAL AERIAL SYSTEMS I Filed March 1 o, 1954 Y 5' Sheets-Sheet '1 mvN aR April 7, 1959 Filed March 10, 1954 R. F. PRIVETT 2,881,434 DIRECTIONAL AERIAL SYSTEMS 5 Sheets-Sheet 2 I l I l I 1 L INVEVNTOR Pm Fkm/K k vETT 1 959 R. F. PRIVETII; I 2,881,434

DIRECTIONAL .AERIAL SYSTEMS Filed March 10, 1954 5 Shets-Sheet 4 INVEN'TQR w Fkq K RIWTT 1 M 7' :FITfORNEY United States Patent 2,881,434 DIRECTIONAL AERIAL SYSTEMS Roy Frank Privett, Montreal, Quebec, Canada, assignor to The General Electric Company Limited, London,

England Application March 10, 1954, Serial No. 415,234

Claims priority, application Great Britain March 13, 1953 11 Claims. (Cl. 343-779) The present invention relates to directional aerial systems and is more particularly concerned with aerial systems of the kind which includes a primary radiator and a secondary radiator.

A primary radiator is one to which radio frequency signals to be transmitted are supplied or which picks up received radio frequency signals and in the present specification comprises at least two radiator elements. A secondary radiator is a purely passive radiator such as a reflector or lens and, if a reflector, it may for example be paraboloidal or parabolic in shape.

A simple construction of directional aerial system comprises a paraboloidal reflector and an open-ended waveguide having its open end facing the reflector and being located in the proximity of the focus of the reflector. This open-ended waveguide acts as a feeder for the aerial system. The main lobe of the radiation pattern of such an aerial system is relatively narrow. For direction finding it has, however, been proposed to provide a receiving aerial system with two waveguide feeders having their open ends adjacent. Considering now these two feeders being used separately in conjunction with the reflector, the main lobe of the radiation pattern of the aerial system using one of the feeders is displaced through a small angle from the corresponding lobe using the other feeder. If now the signals picked up by these two waveguide feeders are compared, the aerial system becomes considerably more directional since it is possible to use a null method. For this purpose it has hitherto been usual to extend these two feeders to an arrangement which compares the two signals and which is located some distance away from the focus, for example behind the reflector (that is to say on the side of the reflector remote from the open ends of the waveguide feeders).

These relatively long feeders are undesirable and it is one object of the present invention to provide an improved construction of directional aerial system in which their length is very appreciably reduced.

According to the present invention, a directional aerial system comprises a primary radiator and a secondary radiator such as a concave reflector, the primary radiator being a device located in the proximity of the focus of the secondary radiator and providing a pair of open-ended waveguides the open ends of which are facing the secondary radiator while the device is adapted to supply a difference signal obtained by comparing the signals picked up by the said pair of waveguides when the aerial system is being used for reception.

Preferably the device comprises a hybrid waveguide junction (sometimes known as a magic-T), the ends of two co-linear arms of the junction each being turned through 90 to provide the said pair of open-ended waveguides. A signal obtained from the said junction by adding the signals picked up by the said pair of open-ended waveguides may be used as a reference signal against which the said difference signal is compared by means of a phase-sensitive detector. If the waveguides forming the junction are of rectangular cross-section, each of the said co-linear arms is turned about an axis parallel to one pair of sides and perpendicular to the other pair of sides of the waveguide forming that arm.

A directional aerial system according to a feature of 2,881,434 Patented Apr. 7, 1959 2 r the present invention comprises a primary radiator and a secondary radiator such as a concave reflector, the primary radiator being a device located in the proximity of the focus of the secondary radiator and comprising three hybrid waveguide junctions, two of these junctions having the ends of co-linear arms each turned through thereby providing two pairs of open-ended waveguides which lie side by side with their open ends facing the secondary radiator and the third waveguide junction being arranged to supply a difference signal obtained by subtracting two signals supplied by the said two junctions respectively, these two signals being derived by those two junctions by adding together the signals picked up by the said pairs of open-ended waveguides respectively when the aerial system is being used for reception.

The waveguides forming a waveguide junction in an aerial system in accordance with the present invention, or each of those junctions if there are more than one, may be air-filled or filled with any suitable solid dielectric material.

Three constructions of aerial system in accordance with the present invention will now be described by way of example with reference to the twelve figures of the accompanying drawings. In the drawings,

Figure 1 shows diagrammatically a complete aerial system while Figure 2 is an explanatory diagram.

Figure 3 shows diagrammatically a conventional hybrid waveguide junction while Figure 4 shows diagrammatically part of the first construction of aerial system which is derived from the junction of Figure 3.

Figures 5 and 6 show the part of Figure 4 in more detail, Figure 6 being a view in the direction of the arrow VI in Figure 5.

Figure 7 shows a perspective view of the part of Figures 5 and 6 together with an associated length of waveguide.

Figures 8 and 9 show an alternative part used in the second construction of aerial system, Figure 9 being a view in the direction of the arrow IX in Figure 8.

Figures 10, 11 and 12 show another form of this part as used in the third construction of aerial system, Figures 11 and 12 being views in the direction of the arrows Xi and XII in Figure 10 respectively.

Figure l of the accompanying drawings shows diagrammatically a directional aerial system in accordance with the present invention which comprises a paraboloidal reflector 1 and a device 2 which has two open-ended waveguides 3 and 4 directed towards the reflecting surface 1. If now the waveguide 3 is considered as the sole feeder of the aerial system, it will be realised this waveguide feeder 3 in conjunction with the reflector 1 gives the system the polar diagram 6 of Figure 2 (only the main lobe of this polar diagram is shown) in the plane of Figure 1. Similarly the feeder 4 in conjunction with the reflector 1 has an associated polar diagram 5 which is angularly displaced through a small angle from the polar diagram 6.

If now this aerial system is used for receiving signals transmitted from a remote source, the amplitude of the signals picked up by the waveguide feeders 3 and 4 will depend upon the angular position in the plane of Figure 1 of the aerial system relative to that source. From Figure 2 it will be seen that these two signals will only be equal when the source is in the direction of the arrow 7. In other words, in the plane of Figure 1, the remote source must be in the directionof the arrow 8 for the signals picked up by the waveguide feeders 3 and 4 to be equal. For the purpose of comparing these two signals, the device 2 is adapted to subtract the two signals and to supply the resultant signal over a waveguide (not 2 shown in Figure l).

1n the first two constructions of aerial system to be described in accordance with the present invention, the device 2 is derived from a conventional hybrid waveguide junction and can best be understood by first considering the operation of a hybrid waveguide junction.

Figure 3 shows such a waveguide junction which comprises two co-linear arms 11 and 12 of equal length and two other arms 13 and 14. It is well known that if signals represented by A and B are supplied to the arms 11 and 12 respectively the signal supplied by the arm 13 may be represented vectorially by A-B W while that supplied by the arm 14 may be represented by The first construction of the device 2 is eflectively formed by turning the arms 11 and 12 of the junction shown in Figure 3 through 90 about the axes 15 and 16. The resulting junction is shown diagrammatically in Figure 4 and it will be seen that the parts 11a and 12a are now parallel to one another and these parts in fact constitute the open-ended waveguides 3 and 4 of the device 2. Clearly the relative sign of one of the signals A or B is now reversed since the signals picked up by the two parts 11a and 12a will have their electric fields in the same direction so that the signals supplied by the arms 13 and 14 may now be represented by A+B A-B v2 In the first construction of aerial system in accordance with the present invention, the four arms of the waveguide junction are provided by a suitably-shaped insert 21 of polythene in a brass block 22 as shown in Figures 5 and 6. The parts 23 and 24 of the insert 21 constitute the open-ended waveguide feeders of the device so that, if A and B represent the signals picked up by these two parts, signals proportional to A--B and A+B are supplied through the parts 25 and 26 respectively. The parts 25 and 26 of the insert 21 are finished flush with the surface of the block 22 seen in Figure 5 and the surface 27 respectively. In fact the brass block 22 is built up from two members which join at a plane parallel to that of Figure 5, and these two members are accurately machined to receive the insert 21 which is itself made up of a number of accurately machined polythene members.

Another waveguide having a resistive load may be clamped to the surface 27 for the purpose of absorbing the signal supplied through the part 26. This waveguide may be of known construction in which one end is closed and the waveguide is filled with lossy material. In practice it is, however, found that this additional waveguide is not essential.

The device 2 has a polythene part 28 which acts as a matching plate between the arms of the waveguide junction and free space, the part 28 effectively forming a continuation of the brass block 22.

In Figure 7, a waveguide 29 of rectangular cross-section is shown clamped to the said surface of the block 22 that is flush with the part 25. The waveguide 29 is filled with polythene and thus forms an extension of the part 25. The waveguide 29 passes over the top of the reflector 1 (Figure l) and acts as a support for the device 2, the flange 35 of the waveguide 29 being suitably mounted. Alternatively the waveguide 29 may pass through an aperture in the reflector 1.

The device is mounted with its phase centre at or close to the focus of the reflector 1. In practice the device 2 is mounted so that this focus lies approximately in the plane dividing the part 28 and the block 22,

and

respectively The difierence signal supplied over the waveguide 29 is fed to a null detector (not shown) which is located behind the reflector 1. This detector may comprise a mixer, an intermediate frequency amplifier, and an am plitude detector connected in cascade in that order. The detector is arranged to indicate when the difference signal has an amplitude less than a predetermined small value and accordingly when the aerial system is to be used for finding the direction of a remote source, it is merely necessary to turn the aerial system until such time as the said detector gives a null indication when the source will be in the direction of the arrow 8 in the plane of Figure 1. If the detector is phase-sensitive, it may compare the phase of the difference signal with a reference signal so as to supply an output signal the polarity of which is dependent upon the direction in which the aerial system must be turned in order to bring about a null indication. The reference signal may be the signal which is supplied through the part 26 and which is fed to the detector over another waveguide (not shown).

As previously mentioned, in prior aerial systems using two open-ended waveguide feeders in conjunction with a reflector, it was usual to extend the two feeders behind the reflector for the purpose of comparing the signals picked up thereby. These feeders were accordingly of considerable length measured in wavelengths and any difference in the attentuation or phase shift provided by the two feeders affected the relative magnitude or phase of the signals supplied to the detector for effecting comparison of those signals and thereby reduced the accuracy of the system for direction finding purposes. In the construction in accordance with the present invention described above, this difliculty is largely overcome since the actual waveguide feeders are very much reduced in length, and it will be realised that since the waveguide 9 merely carries the difference signal to the null detector any attentuation or phase shift introduced thereby does not affect the direction finding accuracy of the system.

The second construction of aerial system in accordance with the present invention is similar to that described above except that the waveguide junction forming the device 2 is somewhat different. This device may again be derived theoretically from the hybrid waveguide junction shown in Figure 3 by turning the arms 11 and 12 through about the axes 31 and 32 so that the two turned parts are parallel to one another. In this case it is not necessary to reverse the sign of either of the signals A or B and the difference signal is accordingly supplied by the arm 13.

The construction of this waveguide junction is shown in more detail in Figures 8 and 9. This device is again formed by a polythene insert 33 in a brass block 34 and the signals carried by the several arms of the junction are as indicated in these figures.

The two constructions of aerial system so far considered are only suitable for direction finding in one plane. The third construction has a more elaborate device 2 (Figure l) which provides four open-ended waveguides facing the reflector 1. Thus, referring now to Figures 10, l1 and 12, these four waveguides are formed by parts 41, 42, 43 and 44 while two waveguide junctions 45 and 46 provide the parts 41 and 42 and the parts 43 and 44 respectively, the junctions 45 and 46 being of identical construction to the junction previously described with reference to Figures 8 and 9. It now the signals picked up by the four parts 41 to 44 are represented by A, B, C and D respectively, the waveguide arm 47 of the junction 45 supplies a signal proportional to AB while the waveguide arm 48 of the junction 46 supplies a signal proportional to CD. Similarly the waveguide arms 49 and 50 supply signals proportional to A-l-B and C+D to a waveguide junction 51. This junction 51 is as described above with reference to Figures 5 and 6 except that the parts 23, 24 and 28 shown in those figures are now omitted. The waveguide arms 52 and 53 of the junction 51 accordingly supply signals proportionlal to (A+B)-(C+D) and A+B+C+D respective y.

In an alternative construction, the junctions 45 and 46 may be as previously described with reference to Figures 5 and 6 while the junction 51 is similar to the junction described with reference to Figures 8 and 9.

Connecting waveguides (not shown) are coupled to the waveguide parts 47, 48, 52 and 53 of this device. The signal supplied by the waveguide arm 52 is fed to a first null detector, and it will be realised that by turning the aerial system so that the device is turned in the plane of Figure 12 the system may be used for direction finding in that plane. Similarly the signals supplied by the waveguide arms 47 and 48 are combined to give a signal proportional to (A+C)(B+D) and this combined signal is fed to a second null detector. It will be appreciated that summing two signals in this manner at a point remote from the device 2 will not introduce such serious inaccuracies as taking the difference of two signals at the remote point in prior arrangements. This second detector may be used for finding the direction of a remote source when the aerial system is turned so that the device is rotated in the plane of Figure 10. In other words, the two null detectors, which are both located a distance from the device 2, may be used independently to determine the elevation and azimuth of a remote source respectively; The aerial system may thus be used to obtain a direction in space of a remote source with a high degree of accuracy.

Although the present invention provides aerial systerns that are highly directional when used for reception it is to be understood that the aerial systems may also be used for transmission. The last-described construction is particularly convenient for use in a radar system having a common aerial system for transmitting and receiving. In that case the signal to be transmitted is supplied from the transmitter to the waveguide arm 53 and it will be realised that one quarter of that signal (neglecting losses) will then be fed to each part 41 to 44. Transmit/receive switches are provided in known manner to prevent any appreciable amount of energy from the transmitter reaching the said null detectors directly through the waveguide junctions.

In the construction of aerial system described above with reference to Figures 1, 10, 11 and 12, the signal supplied during reception from the waveguide arm 53 may constitute the reference signal for the said two detectors and/ or it may be utilised for automatic gain control.

I claim:

1. A directional aerial system comprising a primary radiator and a secondary radiator which has a focus, the primary radiator being a device located in the proximity of thefocus of the secondary radiator and comprising three magic T hybrid waveguide junctions, two of these junctions having the ends of co-linear arms each turned through 90 thereby providing two pairs of openended waveguides which lie side by side with their open ends facing the secondary radiator and the third waveguide junction being arranged to supply a difference signal obtained by subtracting two signals supplied by the said two junctions respectively, these two signals being derived by those two junctions by adding together the signals picked up by the said pairs of open-ended waveguides respectively when the aerial system is being used for reception.

2. A directional aerial system according to claim 1 wherein the waveguides forming each of the said waveguide junctions are filled with solid dielectric material.

3. A directional aerial system according to claim 1 wherein the secondary radiator is a concave reflector.

4. A directional aerial system according to claim 3 wherein the said reflector is paraboloidal in shape.

5. A directional aerial system comprising a primary radiator, a secondary radiator having a focus, and a magic T waveguide junction device which is located in the proximity of the focus of the secondary radiator and which has a pair of open-ended waveguide arms that are co-linear at the junction each having a bend so that the open ends thereof face the secondary radiator and constitute the said primary radiator, the waveguide junction device being filled with solid dielectric material so that it is of appreciably smaller dimensions than would be possible if the device were to have air as dielectric.

6. A directional aerial system according to claim 5 wherein the secondary radiator is a concave reflector.

7. A directional aerial system according to claim 6 wherein the said reflector is paraboloidal in shape.

8. A directional aerial system comprising a reflector which is paraboloidal in shape and which has a focus and a magic T waveguide junction device which is located in the proximity of the focus of the reflector and which has four waveguide arms of which the longitudinal axes of two of the arms are co-linear at the junction and the longitudinal axes of the other two arms at the junction are perpendicular to the longitudinal axes of the co-linear arms and are perpendicular to one another, the two arms that are co-linear at the junction being open-ended and each having a 90 bend so that the open ends thereof face the reflector, these two arms lying symmetrically on either side of a plane that contains the longitudinal axes of the other two arms at the junction, the waveguide junction device being filled with solid dielectric material so that it is of appreciably smaller dimensions than would be possible if the device were to have air as dielectric.

9. A directional aerial system according to claim 8 wherein all the said waveguide arms are of rectangular cross-section, the bend oin each of the waveguide arms that have their longitudinal axes co-linear at the junction resulting in that waveguide arm being turned about an axis that is parallel to one pair of sides and perpendicular to the other pair of sides of the waveguide forming that arm.

10. A directional aerial system according to claim 8 wherein all the said waveguide arms are filled with solid dielectric material.

11. A directional aerial system comprising a reflector which is paraboloidal in shape and which has a focus and a waveguide junction device which is located in the proximity of the focus of the reflector and which has a pair of open-ended waveguide arms that have their longitudinal axes along the lengths thereof lying in a plane so that the open ends of the arms face the said reflector and another waveguide arm that has its longitudinal axis at the junction perpendicular to the said plane, the two waveguide arms of the said pair having the same length and lying symmetrically on either side of a plane which is perpendicular to the previously mentioned plane and which contains the longitudinal axis of the other waveguide arm at the junction so that, when the aerial system is being used for reception, the signal passed to the said other waveguide arm is the dilference between the signals picked up by the open ends of the two waveguide arms of the pair and the waveguide junction device being filled with solid dielectric material so that the longitudinal axes of the pair of waveguide arms are more close 1y spaced than would be possible if the device were to have air as dielectric.

References Cited in the file of this patent UNITED STATES PATENTS 2,585,173 Riblet Feb. 12, 1952 2,632,809 Riblet Mar. 24, 1953 2,677,055 Allen Apr. 27, 1954 2,682,656 Phillips June 29, 1954 

